In recent years, many studies have been actively made on an organic thin-film light emitting device that emits light upon recombination of electrons injected from a cathode and holes injected from an anode in an organic light emitting body interposed between both the electrodes. The light emitting device has been attracting attention because the device is thin and emits light with high luminance under a low driving voltage, and multi-color emission can be obtained by selecting light emitting materials.
When a voltage is applied to an organic electroluminescence device (hereinafter referred to as “organic EL device”), holes and electrons are injected into a light emitting layer from an anode and a cathode, respectively. Then, the holes and the electrons thus injected recombine in the light emitting layer to form excitons. At this time, singlet excitons and triplet excitons are produced at a ratio of 25%:75% according to the statistics theorem of electron spins. When the organic EL devices are classified by their light emission principles, the internal quantum efficiency of a fluorescent organic EL device is said to be at most 25% because the device uses light emission based on a singlet exciton. On the other hand, it has been known that as a phosphorescent organic EL device uses light emission based on a triplet exciton, its internal quantum efficiency reaches 100% when the intersystem crossing from a singlet exciton is efficiently performed.
The device design of organic EL devices has been optimized according to the emission mechanisms of fluorescent devices and phosphorescent devices. In particular, it has been known that a high-performance phosphorescent EL device is not obtained by merely applying the fluorescent device technology because of the difference in their emission properties. The reason for this is generally considered to be as described below.
First, the phosphorescent emission is light emission from a triplet exciton and therefore a compound to be used in the light emitting layer must have a large energy gap. This is because that the energy gap of a certain compound (hereinafter also referred to as “singlet energy”) is generally larger than the triplet energy of the compound (energy difference between the lowest excited triplet state and the ground state).
Therefore, to confine the triplet energy of a phosphorescent emitting dopant material efficiently in the device, first, a host material having a larger triplet energy than the triplet energy of the phosphorescent emitting dopant material must be used in the light emitting layer. Further, an electron transporting layer and a hole transporting layer must be provided adjacent to the light emitting layer, and a compound having a larger triplet energy than that of the phosphorescent emitting dopant material must be used in each of the electron transporting layer and the hole transporting layer. Therefore, according to the organic EL device design conventionally employed, a phosphorescent organic EL device using a compound having a larger energy gap than that of a compound used in fluorescent organic EL devices is resulted, this increasing the driving voltage of entire organic EL device.
In addition, a hydrocarbon compound having high oxidation resistance or high reduction resistance that has been useful in a fluorescent device has a small energy gap because of its wide distribution of π-electron cloud. Therefore, such a hydrocarbon compound is hardly selected in the phosphorescent organic EL device and an organic compound containing a heteroatom such as oxygen or nitrogen is selected instead. However, the organic compound containing a heteroatom shortens the lifetime of phosphorescent organic EL device as compared with that of fluorescent organic EL device.
Further, the device performance is largely affected by the fact that the exciton relaxation rate of a triplet exciton of the phosphorescent emitting dopant material is extremely longer than that of a singlet exciton. Since the relaxation of a singlet exciton which causes emission is fast, the diffusion of the exciton into layers adjacent to the light emitting layer (such as a hole transporting layer and an electron transporting layer) hardly occurs and the efficient light emission is expected. On the other hand, since the emission from a triplet exciton is a spin-forbidden process, and therefore, the relaxation causing the emission is slow, the exciton is apt to diffuse into the adjacent layers, thereby causing the thermal energy deactivation of the exciton, although some specific phosphorescent emitting compounds lead to different results. Therefore, the control of the recombination zone of electrons and holes is more important, as compared with fluorescent organic EL devices.
For the reasons described above, the development of a high-performance phosphorescent organic EL device needs the material selection and device design which are different from those for the fluorescent organic EL devices.
One of the most important problems to be solved in the organic thin-film light emitting device is the compatibility between high emission efficiency and a low driving voltage. To obtain a high-efficiency light emitting device, it has been known to form a light emitting layer by doping a host material with a several percent of a dopant material (Patent Document 1). The host material is required to have a high carrier mobility, a uniform film formability, etc. and the dopant material is required to have a high fluorescent quantum yield, a uniform dispersibility, etc.
A fluorescent (singlet light emission) material has been generally used as the dopant material. To improve the emission efficiency, the use of a phosphorescent (triplet light emission) material has been attempted, and a group of Princeton University has reported that the phosphorescent material provides much higher emission efficiency than obtained by the fluorescent material (Non-Patent Document 1). Many techniques for using a metal complex containing a central metal such as iridium, osmium, rhodium, palladium, and platinum as the phosphorescent dopant material have been disclosed (Patent Documents 2 to 4). As to the host material to be combinedly used with the phosphorescent dopant material, the techniques of using a carbazole derivative, an aromatic amine derivative, a quinolinol metal complex, etc. have been disclosed (Patent Documents 2 to 6). However, a sufficient emission efficiency and a low driving voltage has not been obtained by none of the proposed materials.
A technique of using a biscarbazole derivative as a hole transporting material of a fluorescent device has been disclosed (Patent Document 7). Some patent documents disclose a technique of using a biscarbazole derivative as a host material of phosphorescent device. For example, Patent Document 8 describes a biscarbazole derivative as a host material to be combinedly used with a specific metal complex dopant. However, a high light emission is not obtained by the disclosed biscarbazole derivatives. Patent Document 9 describes the use of a biscarbazole derivative as a host material, in which a substituent for improving the carrier transporting ability of the host material, such as an amino-substituted phenyl group, a naphthyl group, or a fluorenyl group, is introduced into the N-position of a carbazole structure. Although the driving voltage of a light emitting device is reduced by the proposed biscarbazole derivative, its effect on the lifetime is unclear.